Method and apparatus for separating mixtures



Sept. 9, 1947.

J. R. BOWMAN 2,427,042

METHOD AND APPARATUS FOR SBPARATING IIXTURES Filed April 55, 1946 b8SOLVENT RESERVOIR swan/Mow JOj'iN BOWMAN Patented Sept. 9, 1947 METHODAND- APPARATUS FOR SEPA- RATING. MIXTURES John R. Bowman, Pittsburgh,Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., acorporation of Delaware Application April 3, 1946, Serial No. 659,413

Claims. (01. 196-18) This invention relates to a method and app a tusfor separating mixtures; and, more particularly, to a method and.apparatus for separating homogeneous mixtures of solids into theircomponent parts by stratifying the components.

The separation of homogeneous mixtures of solids has .been accomplishedinvarious ways, depending upon the properties of the particular mixturesto be separated. These methods include distillation, extraction,absorption, adsorption, fractional recrystallization, etc. It issometimes possible, by altering temperature or pressure, to separatemixtures by means not applicable to separating the mixture at roomtemperature and atmospheric pressure. For example, mixtures of normallysolid materials may sometimes be separated by distillation, if thesolids are first fused.

The present invention is concemedwith homogeneous mixtures of solids. Asused herein, the term solid will include not only materials which aresolid at room temperature but those which become solid at temperaturesas low as it is practicable to obtain in the apparatus.

In certain applications, prior art separation methods are inadequate.For example, in the .case of the separation of the components ofparaflln wax, which is amixture of a large number of hydrocarbons, priorart methods have been laborious and unsatisfactory.

for separating homogeneous mixtures of solids by recrystallization orresolidification.

The present invention relates to a'process for the separation ofhomogeneous mixtures of solids which comprises first converting thesolids into fluids and passing the fluids through a bed of space-fillingfiltering material. The filter bed is initially subjected to atemperature distribution or spread decreasing in the direction of flowof the fluid and beginning at the inlet end. of the filter bed.Subsequently this temperature distribution is caused to move in thedirection of is caused to move in the direction of flow, for example, byheating the upper end of the filter bed and cooling thelower end so thatthe +20 F. temperature moves gradually in the direction of flow andcontinues to be spaced from the F. temperature by 40 inches. Thetemperature dis- ,tribution extends from a temperature at least highenough to convert the solids to fluids to a temperature atleast lowenough to re-convert the fluids to solids.

The mixture may be carried through the bed of filtering material bygravity, or a solvent may be used and the mixture dissolved in andrecrystallized from the solvent. In this case the separation dependsupon the solubility of the various components of the mixture in thesolvent. If no solvent is used, the mixture may be melted andresolidified. In this case the degree, of separation depends upon theproximity of the freezing points of the various components in themixture.

The preferred apparatus comprises a vertical column packedwith aspace-filling filter medium, such as sand, with means for establishing avertical temperature distribution in the filter medium. This means forestablishing a temperature distribution in the filter medium may be aheating element surrounding the packed column, so arranged that theheating element may be lowered down the length of the column. Thetemperature distribution may require a cooling element provided at adesired distance from, or adjacent to, the heating element. For example,an electric heating element surrounding the column may rest on a, floatupon a body of liquid coolant also surrounding the packed column and theliquid coolant may be withdrawn gradually, thus lowering the heatingcoil.

' A specific embodiment of the invention is illustrated in the drawingin which there is shown a vertical-column I packed with a filter medium2, such as sand. A cylindrical jacket 3 is concentrically arranged aboutcolumn I. In the space between jacket 3 and the wall of column I thereis a movable heating element 5 which floats upon a body of liquidcoolant i. The coolant is pumped by pump 21 from a reservoir 1 throughconduit 8 to a long manifold of valves 8. The valves are adapted to bemanually controlled so that only i one permits entry of coolant into thejacket at any one time, and that one is the one just below the electricheating element. The position of the fioatins. The liquid coolant leavesthe cooling Jacket by means of conduit Ii and may be circulated bypumpI! to a heat exchanger If. The

' coolant passes through coils ll in the heat exchanger li. which inthis instance is packed with dry ice, so that the liquid coolant has itstemperanot shown, to remove any acid formed in the decomposition of sucha coolant as trichlorethylene. The column i is equipped at the topthereof with any conventional means, not shown, for introducing themixture of solids to be separated.

In the apparatus illustrated, there is a solvent circulatory system.There is provided a solvent reservoir I! from which the solvent passesby means of conduit I! and pump ll into the top of the column I. Solventcontaining a separated portion of the mixture is collected by funnel 1|in column I and passes by conduit 2| to a continuous strip r still22,1rom which the desired fractions are removed in turn through conduit23 and the solvent overhead is passed by conduit 24 to a condenser 25wherein it is condensed and returned to solvent reservoir by conduit 20.Also, the lower end of column i may contain a funnel II for collectingsolvent, the constricted portion of which may be heated to preventdeposition of solids and hence avoid plugging up.

In one example of an apparatus of the invention, a small unit forlaboratory purposes, column I had a 6-inch diameter and was 10 feethigh. It was packed with to 60 mesh Ottawa sand and enclosed in aconcentric Jacket 14.5 inches in diameter.

One application of the apparatus is the separation of parafiln wax intoclose-cut fractions of nearly pure hydrocarbons. In one method for doingthis, pentane was used as the solvent for a refined paraiiln wax of 135F. American melting point. Trichlorethylene was used as the liquidcoolant, and its temperature was controlled by passing it through a DryIce heat exchanger. The trichlorethylene at the beginning of theoperation substantially filled Jacket 3 and was slowly bled ofl, so thatits level was continuously lowered over the period of therun. Theelectric heater which floated on top of the trichlorethylene wasconsequently also lowered.- The unit was started by filling the sandpacked column with pentane to the level of the' top of the sand. Aquantity of wax was introduced into the column above the sand and theheater turned on to give the sand in the neighborhood of the heater atemperature of about 20 F., the trichlorethylene being maintained at atemperature of about --65 1''. Circulation of the pentane was startedand the wax which dissolved in the pentane began to pass do the columnin solution. The wax was preciplt ed from solution on the sand as thesolution flowed into a colder region. As the temperature was raised inthat region by the lowering of the heater; the solid wax wasredissolved, fiowed downward, and reprecipitated in the same manner at alower point in the column. Each time the wax was deposited, however, itwas deposited non-uniformly, depending upon the solubili y-temperaturerelationship in the solvent for ea or the components of the wax; themore sol- 4 in the column, i. e., at a lower temperature, than the lesssoluble ones. As the coolant level and the heater continued to lower,the redissolvereprecipitate cycle repeated an indefinite number oftimes, increasing the non-uniformity of the composition distribution.The separation became more and more distinct as the process continued.when the solvent coming out at the bottom of the column contained someportions '01 wax, the solution was passed to the stripper still wherethe solvent was separated from the wax components,- the wax componentsbeing taken 08 in succession from the bottom of the still. Thesecomponents had the characteristics shown in the table below. v

Table I Per cent by cumumin' Molecular Melting wt. Charge 3,? by WeightPoint F. 0. 85 0. 85 50 i 1.42 2. 27 28s 98. 1 1. 57 3. 84 294 107. 1 2.53 6. 37 299 108. 7 5. '12. 12 812 114. 4 4. 10 16. 22 319 116.1 4. 4120. as 326 117. 9 4. 21 2A. 84 329 119. 3 5. 01 29. 8B 335 123. 8 4. 1333. 98 344 127. 4 5. 41 3. 39 352 129. 7 4. 43 43. 82 355 130. 8 3. 7347. 55 355 131. 9 4. 00 51. 66 353 131. 9 2. 82 54. 37 358 132. 3 4. ,658. 63 365 135. 1 2. 73 61. 36 371 136. 9 4. 21 65. 57 377 138. 9 3. 3368.00 382 140. 6 4. 26 73.16 389 142. 7 4. 81 77. 07 404 144. 1 3. 4481. 41 409 146. 0 3. 36 84. 71 I 412 145. 6 5. 63 90. 40 416 146. 1 4.1894. 58 417 146. 1 3. 33 W. 91 41B 146. 3 2. 11 100. 0 417 146. 3

and other properties, the separation of which has usually been carriedout by fractional crystallization. Another example is the concentrationof radium salts from pitoh-blende or other radium containing ores. Stillanother example is the production of pure fatty acids from commercial ortechnical grade products. Thus, in general,

the apparatus and process relate to the separation of homogeneousmixtures of solid materials or materials which are capable of becomingsolid within the particular operating temperatures of the apparatus, andis particularly adaptable to those separations difiicult or impossibleby thev invention to the separation of parafiln wax, the

range of temperature extremes is given as from +20 F. to 65 F. Thistemperature range may be varied by using different cooling means or adifferent amount of heat, and such a temperature range should beadjusted to the materialsubleeomponents of the wax were deposited lowerbeing separated. The efiicienc is increased as ass-1,049

the column.

The filtering material. should hold the solid precipitate at its placeof formation against the motion of the fluid stream until it isredissolved or remelted. Further it should allow the flow of solution ormelted material in a manner which prevents plugging up the apparatus. Ththeoretioally perfect packing is, of course, not obtainable, but inseparations of paraffin was: a good grade of sand has been foundsuillciently eifective.

For a given" temperature gradient, the ratio oflinear solvent flow todistribution travel rate (that is, the rate at which the coolant levelis lowered) should be adiusted to prevent plu ging and to secure thehighest possible efficiency. In the separation of paraflln, it has beenfound that for a temperature gradient of 1 F. per inch, the ratio ofsolvent linear velocity to temperature distribution travel rate should.preferably be kept above 5 to 1. The preferable rate for temperaturedistributiontravelwas found to be about V4 inch per hour for theseparation of paraffin wax, using sand as packing.

An alternative method of causing the temperature distribution to travelalong the column other than by the mechanical means illustrated is toprovide a column which is very cool at one end, preferably the lowerend, and warm enough at the other end to melt or-dissolve the mixture.The entire column may be then gradually heated pausing the usefultemperature distribution range to move toward the colder end of thecolumn. For example, if a given mixture melts at 100 F. and atemperature gradient of 1 F. per inch for inches of the column isrequired to effectively separate the mixture, the lower end of thecolumn may be initially. cooled to a temperature very much lower thannecessary to reconvert the fluids to solids, say 0 F. for a 100-inchcolumn. If the mixture requires 10 inches and a 10 temperature drop toresolidify, then only the top 10 inches of the column would be used forthe first solidification. However, if the entire column is graduallyheated uniformly throughout its length. the point in the column wherethe temperature is 100 F. will gradually be lowered until it reaches thebottom of the column, at which time the whole mixture will have passedthrough the column.

To convert the mixture of solids to a fluid may be done in any one ofseveral ways. For example, as illustrated in the case of the parafhnseparation, the mixture may be dissolved in a solvent. Alternatively itis sometimes possible to melt the solids into liquids, or it is possiblein the case of some solids to transform them into vapors or gasesdirectly as in the case of a sublimed material. I

Various adaptations of the process will be apparent to those skilled inthe art. For example, the process may utilize various heat exchangemethods and devices such as the use of finned tubes. Also it may bedesirable in certain types of filter beds to provide an agitator tobreak up all agglomerates of solids which might form. These and otheradaptations of the invention to particular problems will be obvious tothose skille in the art.

What I claim is: 4

l. A process for the separation of homogeneous I mixtures of solidswhich comprises converting said wuss into sums and passing an fluidsthrough a bed-of space-fillinghltering material, subjecting the bed to atemperature distribution decreasing in the direction of how andbeginning" at the inlet end of the bed, and subsequently causing thistemperature distribution to move in the direction of flow toward theoutlet end of the bed and recovering successive fractions of theintroduced mixture of solids, said temperature distribution. extendingfrom a temperature at least high enough to convert the solids to fluidsto a temperature at least low enough to reconvert the fluids to solids.

2. Aprocess for the separation of homogeneous mixtures of solids whichcomprises melting said solids and passing said melted material through abed of space-filling filtering material, subjecting the bed to atemperature distribution decreasing in the direction of flow andbeginning at the inlet end of the bed, and subsequently causing this.temperature distribution to move in thedirection of flow toward theoutlet end of the bed and recovering'successive fractions of theintroduced mixture of solids, said temperature distribution extendingfrom a temperature at least high enough to melt the" solid to atemperature at least low enough to resolidify the solids.

3. A process for the separation of homogeneous mixtures of solids whichcomprises vaporizing said solids and passingv said vaporized materialthrough a bed of space-filling filtering material, subjecting the bed toa temperature distribution decreasing in the direction of flow andbeginmm at the inlet end of the bed, and subsequently causing thistemperature distribution to move in the direction of flow toward theoutlet end of the bed and recovering successive fractions of theintroduced mixture of solids, said temperature distribution extendingfrom a, temperature at least high enough to vaporize the solids to atemperature at least low enough to resolidify the solids.

4. A process for the separation of homogeneous mixtures of solids whichcomprises dissolving said solids andpassing the resulting solutionthrough a bed of space-filling filtering material, subjecting thebed toa temperature distribution decreasing in the direction of flow andbeginning at'the inlet end of-the bed, and subsequently causing thistemperature distribution to move in the direction of flow toward theoutlet end of the bed and recovering successive fractions of theintroduced mixture of solids, said temperature distribution extendingfrom 'a temperature at least high enough to dissolve the solidsto atemperature at least low enough to resolidify substantially all of thesolids.

5. A process for the separation of paraflin wax Q into fractionsrespectively enriched with regard to individual hydrocarbon componentsof the wax which comprises dissolving said paraflln wax and passing theresultant solution through a column packed with sand, subjecting the bedto a temperature distribution of from about +20 F. to about 65 F.decreasing in the direction of flow and beginning at the inlet end ofthe bed, and

subsequently causing this temperature distribu-.

tion to move down the column, while continuous- 1y adding solvent at thetop of the column, so that the paraiiln wax alternately dissolves andreprecipitates out of solution, and distilling fractions of the solutionwithdrawn from the column to obtain said component hydrocarbons.

6. A process for the separation of homogeneous mixtures of solids whichcomprises converting said solids into fluids and passing said fluidsthrough a bed of space-filling filtering material subjecting the bed toa temperature distribution decreasing in the direction of flow andbeginning at the inlet end of the bed, and subsequently causing thistemperature distribution to move in the direction of flow toward theoutlet end of thebed by means of the movement of a heatingunitalongthefllterinsbedprecededbyabody of coolant and recoveringsuccessive fractions of the introduced mixture of solids. said tempera.

ture distribution extending from a temperature at least high enough toconvert the solids to fluids to a temperature at least low enough toreconvert the fluids to solids.

'7. A process for the separation of homogeneous mixtures of solids whichcomprises converting said solids into fluids and passing said fluidsthrough a bed of space-filling flltering material, subjecting the bed toa temperature distribution decreasing in the direction of flow andbeginning at the inlet end of the bed, and subsequently causing thistemperature distribution to move in the direction of flow toward theoutlet end of the bed and recovering successive fractions of theintroduced mixture of solids, said temperature distribution extendingfrom a temperature at least high enough to convert the solids to fluidsto a temperature at least low enough to reconvert the fluids to solids,said temperature distribution being caused to move by gradually heatingthe entire bed of flltering material which was initially cooled at oneend to a temperature very much lower than necessary to reconvert thefluids to solids.

8. An apparatus for separating mixtures of solids comprising a verticalcolumn packed with a space-filling fllter medium, a surroundingconcentric Jacket containing a liquid coolant, heating means in saidconcentric jacket arranged above said liquid coolant, and means forgradually lowering the level of said liquid coolant and said heatingmeans.

9. An apparatus for separating mixtures of solids comprising a verticalcolumn packed with a space-filling fllter medium, a surrounding con-.centric Jacket containing a liquid coolant, heating means in saidconcentric Jacket adapted to float upon said liquid coolant, a series ofvalves along the length of said concentric Jacket adapted to admitliquid coolant at predetermined levels. means for withdrawing saidliquid coolant near the bottom of said concentric jacket and means forremoving heat from saidliquid coolant.

10. An apparatus for separating mixtures of solids comprising a verticalcolumn packed with a space-filling fllter medium, a surroundingconcentric Jacket containing a liquid coolant, heating means in saidconcentric jacket adapted to float upon said liquid coolant, a series ofvalves along the length of said concentric jacket adapted to admitliquid coolant at predetermined levels. means for withdrawing saidliquid coolant near the bottom of said concentric jacket and means forremoving heat from said liquid coolant, means for introducing a mixtureof solids at the upper end of said vertical column, means forintroducing a solvent at the upper end of said vertical column, meansfor withdrawing a solution of solids at the lower end of said verticalcolumn, means for distilling said solution and means for returning saidsolvent from said solution to said vertical column.

' JOHN R. BOWMAN.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Petroleum Reflner, vol. 22, No. 6, May 1943, mes98-102 (article by Turner).

